Fin roll stabilizers are commonly mounted to the hull of a vessel below the waterline, usually within the middle one third of the vessel's waterline length and close to the turn of the bilge. These fins typically rotate about an axis that is perpendicular to the lengthwise axis of the vessel. The stabilizer fins are generally aligned parallel to the lengthwise axis of the vessel and rotation of these fins reduces roll of the vessel. The fin roll stabilizers act in some ways that are similar to ailerons on an airplane.
Many traditional fin roll stabilizers for marine vessels are powered hydraulically. In order to create a functioning stabilizing system, a complex setup of hydraulic plumbing, valves, cylinders and pumps are needed to operate and control the stabilizer. In addition, a reservoir must be provided to supply the pumps with hydraulic fluid and a cooling system and filter is needed to prevent overheating and to keep the fluid clean. The hydraulic system components all need connection with hose or pipe which can run long distances within a vessel and can be difficult to install and require maintenance.
Electrical sensors and controls are then needed to operate various valves within the hydraulic system to allow the stabilizer fins to move through varying degrees of rotation that depend on vessel speed and wave conditions that are causing the vessel to roll.
The hydraulic system also comes with added environmental concerns associated with hydraulic oil because a leak in the system can bleed into the bilge of the vessel and be pumped out with bilge water into the ocean or other body of water where the marine vessel is operating.
The hydraulic fluid is often put under high pressure such that the associated parts within the system must be designed to withstand that substantial pressure. In addition, air in the hydraulic system can cause control problems and when the air bubbles collapse they generate intense localized heat in the hydraulic oil leading to system overheating and early hydraulic component failure. Air in the system can also cause objectionable noise and vibration transmitted throughout the vessel via the interconnecting hydraulic piping. Routine maintenance or servicing of an otherwise closed hydraulic system, such as changing a hose or a valve, can easily introduce debris or contaminants that can cause intermittent problems that are very difficult to diagnose, require complete system flushing and can lead to expensive repairs.
Another concern of a shipboard hydraulic system is fire. At higher pressures the hydraulic system is more prone to leaks and when a leak occurs the oil can mist or spray onto hot surfaces in the vessel's machinery spaces or be vaporized, and when exposed to a source of ignition vaporized oil causes fire.
An alternative to hydraulics has been direct drive electric motors that rotate the fin shaft via a planetary gear set. However, planetary gear arrangements are prone to backlash and positioning errors, and those errors increase as the gears experience wear. Strain wave gear sets eliminate backlash, but are less efficient, are limited in torque capacity and are prone to ratcheting phenomenon in momentary peak conditions (this condition is called dedoidal and can damage the drive).